A dipped beam is characterized by a cutoff, i.e. a generally horizontal upper limit above which no light may be emitted. FIG. 1 of the accompanying drawings shows an example of a cutoff as projected onto a standard screen at a range of 25 meters (m). This specific cutoff is laid down by regulations in several countries, in particular in Europe. For traffic using the right-hand side of the road, this cutoff is constituted by a horizontal half-plane h'H extending leftwards from the horizontal longitudinal axis of the vehicle and by a half-plane Hc extending rightwards from the same axis and sloping slightly upwardly, typically at an angle .alpha.=15.degree.. Naturally, this configuration is inverted for traffic using the left-hand side of the road.
In addition to satisfying this cutoff in order to avoid dazzling drivers of vehicles coming the other way, a dipped headlight beam must satisfy various other requirements concerning light intensity at various points and regions below the cutoff. In particular, the "bright spot", i.e. the region of the illuminated field in which light concentration is at a maximum, must preferably be situated immediately beneath the cutoff and slightly to the right of the central vertical axis v'-v passing through the longitudinal midplane of the vehicle so as to illuminate the side of the road adequately. This concentration is determined, in particular, by measuring the light flux at test points called "75R" and "50R", which flux must be greater than a specified minimum allowable amount.
Conventional dipped headlights have a filament lamp, a reflector, a mask which defines the above-mentioned cutoff, and a spreading glass which closes the headlight, and it is common practice for the desired concentration of the beam as defined to be obtained by making the reflector and the glass with highly specific optical characteristics.
However, Cibie's published French patent application No. 2 536 502 of Nov. 19, 1982 describes a dipped headlight without a mask. More precisely, the above-defined "European" type of cutoff is obtained solely by virtue of special designs for the reflector and the glass. This headlight includes a reflector having two sectors in the form of paraboloids of revolution about a common axis, said sectors being disposed symmetrically about said axis and being delimited by two axial planes, one of which is horizontal and the other of which is at an angle to said horizontal plane equal to the lift angle .alpha. of the dipped beam cutoff. The headlight also has a lamp with an axial filament which is upwardly offset radially from said common axis, and a beam-spreading glass placed in front of the reflector, with the zones of the glass corresponding to said two paraboloidal sectors being arranged to deflect the beam to a small extent only. As described in said published patent application, the two paraboloidal sectors have a common focus situated axially beneath the center of the filament, and they have the same focal length.
The main advantage of such a headlight is a considerable increase in the delivered light flux by virtue of the mask being omitted.
However, the bright spot obtained with this headlight is essentially centered on the longitudinal axis of the vehicle, as can be seen in FIG. 2 of the accompanying drawings, which shows isocandela curves C.sub.i of the illumination produced by such a prior art headlight on a standardized screen at 25 meters (the shaded zone T). This position has two major drawbacks. Firstly, such a central bright spot is extremely sensitive to vertical oscillations of the vehicle. Thus, when the vehicle pitches, very marked differences appear in the illumination of the road ahead, thereby tiring the eyes of the driver. Secondly, in order to optimize the visibility distance given by a dipped headlight, the bright spot should be superposed over the point 75R as defined in the European regulations, i.e. said spot needs to be offset on the screen to the right and upwardly. In this respect, it is difficult, in practice, to offset said bright spot to the right and upwardly by means of deflecting prisms or the like in the headlight glass, since there is considerable degradation in the sharpness of the cutoff with the danger of too many light rays leaking upwardly and dazzling the drivers of oncoming vehicles. Inevitable small manufacturing defects in the molding of the closure glass thus have too great an effect on the final product.
The present invention seeks to improve the maskless dipped headlight in such a manner as to obtain a beam which is not only completely satisfactory as to its cutoff but which also has a bright spot which is properly offset to the right from the longitudinal central axis of the vehicle.